Meaning and mechanism
A CdS photoresistor generally decreases in resistance as illumination increases. The sensor converts a physical quantity into a signal change that a circuit can read.
When CdS is located below a voltage divider circuit, the resistance and output voltage increase as the darkness level increases. By setting a comparison threshold, it is possible to create a control that turns on when the darkness level reaches that threshold.
Relationships and conditions
Insert the sensor’s resistance–illuminance relationship into the voltage-divider equation Vout=VRcds/(R+Rcds) to calculate output. Illuminance and voltage are generally not directly proportional.
Actual sensors exhibit response time, component variation, and temperature effects. To reduce repeated on-off cycles near the threshold, hysteresis and similar considerations can be applied.
What to observe in the simulation
With CdS as the lower resistor, the divider output rises as it gets darker.
A single representative CdS curve is used. Delay, device variation and comparator hysteresis are omitted. Experiment on screen; do not reproduce this with real power supplies, outlets or high voltage.
Key relationships and symbols
Rcds = 10 kΩ(lux/100)^−0.7 · Vs = V Rcds/(R+Rcds)
| Symbol | Meaning | Unit |
|---|---|---|
| Illuminance | Quantity describing the light arriving at a surface | lx |
| Rcds | Photoresistor resistance | Ω |
Worked example
Is the streetlight on with sensor output 4.17 V and a 3.5 V reference?
On
Common misconceptions
- The direction of sensor output increasing when it brightens cannot be determined without verifying the sensor and circuit arrangement.